Although the results of this study may not directly relate to genetic factors contributing to atopic predisposition, two observations merit discussion. First, aside from genetic atopic predisposition, allergen-specific IgG1 provides parsimonious estimates for the genetic factors related to atopy-associated humoral immune responses to non-infectious environmental allergens, as it is not significantly effected by demographic co-factors. Its source is clearly defined, arising from a specific immune response to a defined antigen, whereas the physiological ‘sources’ of total IgE are unclear. Second, the genetic components of atopic sensitization to allergens contribute to a more generalized immune system deregulation influencing antibody isotype production other than IgE.
These findings are in contrast to those of other studies in which only IgE measures were used, particularly those in which it was implicitly assumed that atopic sensitization to allergens is an IgE-related phenomenon only. Using total IgE as a quantitative trait for genetic studies of atopy has produced widely conflicting results, particularly with regard to the proportions of the responses putatively attributed to inheritance [1
]. Although controlled by tight metabolic regulation, the production of IgE involves multiple pathways [26
], but the details of these interrelationships are not completely understood.
Epidemiological studies have shown that serum total IgE levels are significantly influenced by both demographic factors, like age, gender and race, as well as atopic clinical status [29
], which undoubtedly confound the studies of inheritance. This was also observed in our study population, as shown in the results of and . Whereas the IgG1 values reflect immune response capabilities to defined antigens, it cannot be determined with any certainty what the sources are for IgE production.
The measured serum IgE values could conceivably arise from three, possibly interdependent, sources as either basal IgE production unrelated to atopic disease (“non-cognate fraction”), specific IgE directed against one or more allergens (“cognate fraction”) [11
] or elevated IgE production due to atopic respiratory conditions (“ectopic fraction”). Attempting to account for one or more of these factors suggest that IgE production is influenced by multiple genes, some of which may not be related to immune system regulatory elements [24
A comprehensive survey of genetic studies on atopy-related phenotypes showed that no less than 64 genes or chromosomal regions have been implicated in these disorders, of which roughly 50–60% were immune regulatory genes, including inflammatory and antibody production promoters, while the remainder had no apparent association with immune system regulation [34
]. These complex scenarios argue against any obligatory inherited pathway(s) leading to an atopic outcome.
As we have previously argued [12
], the atopic predisposition more likely arises from a “stochastic bias” model. In this scenario, it is equally likely for anyone to mount an immune response to allergens, but the specific outcome is a random event. The specific outcome is dependent upon the interplay of numerous genetic and environmental elements that, in certain circumstances, may “skew” a response towards atopy, while others experiencing similar conditions remain clinically unaffected. A first step toward unraveling this complexity is to understand the commonality of the atopic and non-atopic humoral responses, as opposed to attempting to clearly delineate the presumed differences.
We have previously shown among randomly selected individuals, that those with and without atopic sensitization to a particular allergen can mount an equally vigorous humoral immune response against that allergen [15
]. The atopic response was characterized by the production of specific IgE with very high equilibrium binding affinity for the allergen and an attenuated, lower affinity specific IgG1 response. In contrast, the non-atopic response was characterized by specific IgG1 with an equilibrium binding affinity for the allergen comparable to that of the atopic IgE response.
We have also found that in the earliest years of life, when most atopic humoral responses develop, that there is a non-linear increase in the age-dependent ‘developmental trajectories’ of humoral responses involving specific IgE among atopic children and specific IgG1 among non-atopic children are remarkably similar [35
]. Interestingly, this study also showed that the atopy-associated specific IgG1 response was ‘deregulated’ compared to the IgG1 response among non-atopic children, as there was no comparable non-linear increase with age. Rather, the vigor of the atopic IgG1 response remained relatively unchanged, suggesting again that stochastic features among immune regulators favored a vigorous IgE response at the expense of a vigorous IgG1 response among those children with an atopic predisposition.
From these results we have surmised that atopy may only be an ‘error’ in isotype class switching during an otherwise normal process of antigen recognition and humoral response development. If this is so, then as shown in , about 60% of the factors that contribute to this ‘error’ are inherited. This does not imply, however, that these factors will be common to all people with the atopic predisposition, or that families with similar atopic histories will also share the same genetic complement predisposing to atopy. Again, from the literature survey by Hoffjan et al [34
], there are numerous genes that may contribute to an atopic immune response. By probability alone, it is extremely unlikely that all people with atopy or all families with atopic histories could possibly share the same combinations of these genes.
Recent evidence suggests that single nucleotide polymorphisms (SNP’s) in the promoter regions of immune regulatory genes can profoundly impact upon the specific details of an immune response to allergens [36
]. Three specific examples include SNP’s in the CD14 endotoxin receptor gene that influences both allergen-specific IgE andIgG1 production among those with atopy [37
], SNP’s in the promoter region for ICOS (inducible co-factor of stimulation) that mediates T cell-B cell interaction during humoral response development [38
] and CTLA-4 gene promoter SNP’s that also impacts upon T cell-B cell interaction [39
Given that each of these gene promoter elements can have at least one of three genotypes (e.g. C/C, C/T or T/T for the CD14 promoter), then there are at least 3 x 3 x 3 = 27 possible combinations taking all three together. If only one of these combinations is the “correct” one for atopy, then in the total population there is about a 1/27 ≈ 4% chance of an atopic response, which clearly does not conform to the current atopy incidence rates in industrialized nations of about 25–40%. Of course, any number of the 27 possible combinations could be proposed to be “correct” to account for current atopy incidence, but assuming that any of these combinations will be exactly represented in all atopic families is, again, extremely unlikely.
What is more likely, however, is that IgE and IgG1 production may have more immune regulators in common with one another than are disparate, and a more thorough search among these regulators might provide clues to the probability of isotype switching ‘errors.’ For example, considerable attention has been paid to the genes in the “cytokine cluster” region on human chromosome 5q31 [41
] that includes genes for IL-4, CD14 and the beta-2 adrenergic receptor (β2
-AR) among others.
By focusing upon this region it was hoped that gene alleles would become evident that promote enhanced IgE production, and provide clues to the atopic predisposition. However, the cytokine IL-4 is a critical antibody isotype ‘switch factor’ for both IgG1 and IgE [18
], CD14 modulates both allergen-specific IgG1 and IgE responses [37
] and β2
-AR agonists have been shown to enhance the amount of IgG1 and IgE produced per B cell in vitro
]. Any one alone or in combination could account for enhanced production of both specific IgG1 and IgE.
However, the dynamics of humoral response development are not totally accounted for by isotype switching and the amount of the particular isotype that is produced. Somatic hypermutation of the genes encoding for the variable heavy and light chains of antibodies are also involved, which leads to enhanced antibody binding to antigen during ‘affinity maturation’ in germinal center reactions [42
The fully developed humoral response is a function of both the concentration of the particular antibody isotype, [IgX], and the binding affinity, K, of this antibody for the antigen that induced its production. The mathematical production of these two variables, that we have called the total antibody binding capacity (CAP = [IgX] x K), is the variable that clearly distinguishes the atopic from the non-atopic humoral response [15
]. It is the age-dependent, nonlinear increase in total IgG1 and IgE capacities that are comparable in non-atopic and atopic children [35
Both the atopic and the non-atopic humoral responses are the result of the interactions among numerous genes, that control isotype class switching, the amount of the particular isotype that is produced and factors involved in affinity maturation. It has been argued that the mechanisms of isotype class switching and affinity maturation within lymph node germinal centers are independently regulated [43
]. In effect, the particular antibody repertoire that develops in any given circumstance is a matter of most probable outcomes, akin to Darwinian competitive selection on a microscopic scale [15
]. From the results of the present study, allergen-specific IgG1 is a robust, and convenient, quantitative marker to be used to unravel the genetic complexities of the atopy-associated humoral response to allergens.